Image forming devices are capable of forming images on paper by transferring a toner image onto the paper, and fixing the transferred toner image onto the paper by means of a heating process performed by a fixing unit. The fixing unit is provided with, for example, a heating roller (fixing roller), and a pressure roller. The heating roller and pressure roller abut one another such that the surfaces of the rollers are in mutual contact at a predetermined pressure, and at least one roller (for example, the heating roller) is heated by a heater. According to this configuration, a paper sheet bearing a transferred toner image is subjected to a fixing process, such as a heating process, pressure process and the like, as the sheet passes between the heating roller and pressure roller, so as to fix the transferred toner image onto the paper.
During the image formation operation, the fixing process heating process) can be satisfactorily performed by actuating the heater such that the temperature of the heating roller reaches a predetermined fixing temperature (for example, 170° C.). At times other than during the image formation process, that is, when the operation of forming an image on paper is not being performed (operation of the toner image transfer process, operation of the fixing process and the like), the drive current supplied to the heater is reduced and the temperature of the heating roller is reduced to a predetermined temperature (for example, 135˜165° C.) that is less than the fixing temperature. In this way the power consumption of the fixing unit (heater) can be reduced, and the temperature of the heating roller can be raised to the fixing temperature in a short time when the image forming operation begins.
When a relatively large size paper sheet is used (for example, A3 or B4 sizes), and when a relatively thick (high weight ratio) sheet is used (for example, OHP sheet), the heater has a longer actuation time, and the power consumption of the fixing unit is increased due to the large amount of heat imparted to the sheet by the heating process performed by the fixing unit. When a relatively small size paper sheet is used (for example, B5 or A5 sizes), and when a relatively thin (low weight ratio) sheet is used (for example, tracing paper and the like), the heater has a shorter actuation time, and the power consumption of the fixing unit is reduced due to the small of heat imparted to the sheet by the heating process performed by the fixing unit.
Accordingly, when an image forming operation is performed using only relatively thick (high weight ratio) sheets or relatively large size sheets within a predetermined time period, the power consumption of the fixing unit increases and the power consumption of the entire image forming device increases within the predetermined time period.
Based on this background, an object of the present invention is to provide an image forming device capable of suppressing an increase in power consumption.